1. Field of the Invention
This invention relates to dual-gimbaled optical systems such as roll-nod gimbal optical systems, and more particularly to dual-gimbaled optical system having an offset aperture.
2. Description of the Related Art
A dual-gimballed optical system includes an outer gimbal configured to rotate around a first axis, an inner gimbal mounted on the outer gimbal and configured to rotate around a second axis and an optics assembly mounted on the inner gimbal. Typical dual-gimbal configurations include roll/nod, nod/roll, pitch/yaw or Az/El. The optics assembly has a field-of-view (FOV). The dual-gimbal is configured to slew the FOV to trace out a much larger field-of-regard (FOR). The optics assembly is optically coupled to an optical sensor or an optical source. In some systems, the source or sensor may be positioned on the gimbal. In others, the source or sensor is positioned off gimbal.
As shown in FIGS. 1a through 1c, a dual-gimbaled optical system 10 comprises a roll gimbal 12 that rotates 13 around a roll axis 14, a nod gimbal 16 on the roll gimbal that rotates around a nod axis 18 perpendicular to the roll axis at the gimbal center 19, an optics assembly 20 with its optical collection aperture 21 centered on the nod gimbal and symmetric about both the roll and nod axes and a sensor or source (not shown) optically coupled to the optics assembly. The roll and nod gimbals are shown schematically, each gimbal includes a mechanical rotation axis, a motor to provide rotation about the rotation axis and a controller to control the motor. The optics assembly transmits or receives optical radiation in a FOV 22 along an optical axis 24. The dual-gimbal points optical axis 24 in a multi-dimensional space defined by the first and second axes to slew the FOV over a FOR such as a hemisphere.
When the source or sensor is positioned off-gimbal, light is optically coupled between the source or sensor and the optics assembly via a Coudé path 28. Coudé path 28 includes four folding mirrors 32, 34, 36 and 38 that fold light between the optics assembly and the roll axis to the off-gimbal source or sensor. The first folding mirror 32 may be incorporated in the optics assembly on the nod gimbal. The second, third and fourth folding mirrors may be formed in a prism.
In many systems, the dual-gimbaled optical system is positioned behind a dome 40. The dome is optically transparent to the spectral band received and/or transmitted by the optical system and provides protection for the optical components. The dome has an inner curvature that is symmetric about the roll axis. From an optical aberration correction perspective, the dome should be flat or spherical in shape.
If the dome is non-spherical, highly curved, or thick, the dome may introduce significant wavefront aberration into the optical rays that pass through the dome, particularly at or near the tip 41 of the dome. These domes are typically referred to as a “conformal” dome. Conformal means that the dome conforms to a desired shape for some reason other than optical correction. Conformal domes are neither flat nor spherical. Some examples of conformal shapes include ogive or steep ellipse. Conformal domes present significant advantages in aerodynamic performance, and possibly aesthetics, while presenting greater optical challenges. The conformal dome may be mounted on various platforms such as missiles, airplanes, unmanned aerial vehicles (UAVs), helicopters, terrestrial or sea-based vehicles and may be mounted forward, aft, sideways or on the belly of the platform. The axis of symmetry of the dome may be coincident with or orthogonal to a long axis of the platform.
Conformal domes present significant advantages in aerodynamic performance while presenting greater optical challenges. A transparent optical corrector 42 in the form of an aspheric transparent arch having a shape responsive to the shape of the dome may be placed on the roll gimbal in the optical path between the conformal dome 40 and the optics assembly 20 to compensate for the aberrations introduced by the non-spherical window. The arch is fixed with respect to the nod axis. As the arch rotates with the roll gimbal, the transparent arch is suitably designed to just cover the FOV over all allowed nods of the nod gimbal. An embodiment of the transparent arch corrector 42 is described in detail in U.S. Pat. No. 6,028,712. An embodiment of a “half-arch corrector” is described in U.S. Pat. No. 6,310,730. The “half-arch” corrector is lighter weight than the “arch” corrector but limits the nod range of motion and requires the optical train to roll an extra 180 degrees to cover the nod range in order to cover the same FOR.
In certain missile systems it is required that the FOR look both forwards and down. The optics assembly is typically mounted on a nod/roll gimbal behind a flat window and a conformal dome. To accommodate this requirement, the optics assembly is offset from the nod axis so that the optical aperture is completely off of the nod axis. The aperture is symmetric about the roll axis.
In certain missile countermeasures systems, a large aperture receive optics assembly is mounted on a central region of the nod gimbal symmetric about the roll and nod axis to receive optical radiation from a target. The received optical radiation is coupled to an off-gimbal detector. A small aperture transmit optics assembly is mounted on an offset region of the nod gimbal such that the aperture of the transmit optics is completely offset from both the roll and nod axes. The range of motion in nod is limited due to mechanical clearances of the optics package.